The present invention relates generally to receivers for TVRO earth stations which receive audio and video signals from a plurality of orbiting earth satellites. More particularly, the invention relates to a TVRO receiver system for locating the subcarrier frequencies for multiple audio signals included in the transmissions received from satellites having multiple transponders.
As used herein, the term audio "subcarrier" refers not only to audio carrier frequencies which are secondary to a main video carrier handled by the same transponder, but also to audio carrier frequencies used for FM transmissions in the absence of video carriers.
In satellite communication systems, a transmitting earth station generates a modulated carrier in the form of electromagnetic waves up to a satellite, forming an "uplink." The incident electromagnetic waves are collected by the satellite, processed electronically to reformat the modulated carrier in some way, and retransmitted to receiving earth stations, forming "downlinks." The earth stations in these systems basically consist of a transmitting and/or receiving power station functioning in conjunction with an antenna subsystem and form strategic parts of the satellite communication system.
A TVRO earth station typically comprises a receiving antenna such as a paraboloidal dish, a low noise block converter (or a low noise amplifier (LNA) and a down converter) located at an outdoor antenna site, and a receiver located near an indoor television set. The down converter and the receiver are usually connected by a coaxial cable.
A single transponder in a satellite can carry a color television channel, including both the video and audio information, and also several auxiliary services such as radio stations, newservice feeds, special news teletypewriter channels, high-speed stock market and commodity exchange data feeds, and/or teletext data services. A transponder normally has a usable modulating signal bandwidth of 8 to 10 MHz, and the video information normally occupies the band up to about 4.2 MHz. The audio portion of the television channel is placed on an FM subcarrier in the 5.8 to 7.4 MHz range (usually either 6.2 or 6.8 MHz), which leaves available all the other FM subcarrier frequencies located above 4.2 MHz. For example, in several of the satellites presently orbiting the earth, transponder owners feed separate audio subcarriers at 5.8, 6.2, 6.8 and 7.4 MHz. Other transponders on the same satellites carry music services at 5.58 and 5.76 MHz.
A satellite transponder may also be used to carry multiple narrow-band audio signals in place of a wide-band video signal. These audio signals may be interspersed with other types of auxiliary services, and thus the exact frequency of the audio subcarriers can vary widely among the large number of transponders presently in orbit, and new subcarriers can become available at any time as more subcarrier services are squeezed onto existing transponders and as additional satellites are placed in orbit. Moreover, there is no fixed relationship among the numerous audio signals themselves.
Current satellite receivers often include tunable audio subcarrier detectors for locating the various "hidden" audio subcarriers. Some of these receivers include a tuning knob for selecting the 6.2-MHz or 6.8-MHz subcarriers because these are two of the most commonly used frequencies. Other receivers have a tuning knob that permits the user to manually scan the FM baseband spectrum to locate the various audio subcarriers being received from each transponder; this scanning and tuning procedure must, of course, be repeated for each different transponder, which can become a tedious process when searching for the narrow-band audio signals transmitted by a number of different transponders.